Optical fiber has been well established as a reliable transmission medium. For many uses, a plurality of optical fibers are assembled together protected by sheath components to form a cable structure. Typically, a plurality of optical fibers are assembled in a bundle and one or more bundles are used to form a unit. Alternatively, a cable may include a core which comprises a plurality of optical fiber ribbons, each ribbon including a plurality of optical fibers, arrayed in parallel relationship to each other with the longitudinal axes of the fibers disposed in a plane. Typically, an optical fiber comprises a core and a cladding and one or more coatings.
During a process of manufacturing a glass optical fiber, a glass fiber is drawn from a preform and then coated with one or more coating materials, typically, ultraviolet light curable materials. The coating materials include, for example, polymeric compositions and are applied by one or more coating applicators. The function of the fiber coating is to protect the surface of the glass optical fiber from mechanical scratches and abrasions which the optical fiber may experience during subsequent handling and use. The coating or coatings also influence the fiber's optical characteristics in response to external mechanical forces and environmental temperature.
In a cable, for example, optical fibers are identified from one another by the use of a color coating layer which has been applied to the coated optical fiber. In the past, desired colors have been obtained in commercially available color coatings by using a dispersion of colored pigment particles in a suitable liquid carrier.
The use of pigmented materials to provide color coatings for optical fiber has presented manufacturing and performance problems. For example, pigment particles dispersed in an organic binder resin, as in all physical mixtures, gradually will separate into two distinct phases. As a result, pigmented color coatings have a relatively short shelf life.
The occurrence of phase separation in a pigmented colorant system is complicated by the concurrent agglomeration of pigment particles. Undesirably, the presence of agglomerates of pigment particles in a color coating on a coated optical fiber can induce microbending which results in transmission losses.
Further, the relatively high concentration of pigment material which is required to achieve an opaque ultraviolet light curable color coating inhibits the transmission of incident light which is necessary to cure the color coating material. The pigment constituent refracts, reflects and scatters light from the curing source thereby making it difficult to cure the coating material. This results in a reduction in processing speed of the optical fiber along a manufacturing line and thereby increases production costs. The inherently slow cure speed of pigmented color coatings causes the processing and the cure of these materials to be sensitive to minor changes in the thickness of the color coatings.
An additional drawback to the use of pigmented materials is that some pigments include heavy metals such as lead or cadmium. The use of such constituents in a color coating for optical fiber presents a safety question. Additionally, the use of optical fiber color coated with such pigmented systems in above-ground, outside plant may be deleterious to the environment.
Because of these disadvantages of pigment-based color identification systems, thought has been given to using dyes. However, there is a problem associated with the use of dyes, and this problem relates to some of the coating materials used on optical fibers. Polymeric coatings are effective to prevent mechanical damage to the glass fiber surface they are meant to protect; however, diffusion of water vapor, hydroxyl ions, and hydrogen through the polymeric coatings pose additional threats to the strength, mechanical integrity and optical performance of the optical fiber.
Mechanical failure of the optical fiber may occur through a glass fiber failure mechanism referred to as stress corrosion. In an outer surface of a glass body, there exist surface imperfections resulting from mechanical damage or flawed silica bonds, for example. These imperfections, which are called microcracks, act as stress concentrators and thus may cause failure to occur preferentially at these locations when the fiber is subjected to tensile stresses. As stress is increased to a certain critical level, the fiber will fail at the crack site. Normally, these cracks will not grow under the influence of stress alone. In the presence of contaminates, hydroxyl ions, for example, the source of which may be water vapor, these cracks tend to grow at predictable rates when subjected to tensile loading. This stress corrosion is the result of the incorporation of the hydroxyl ions into the silica matrix of the optical fiber. Fiber failure may occur at stress levels significantly below an otherwise higher level due to the fact that the microcracks slowly but steadily reduce the area over which the tensile loads are resisted.
The presence of hydrogen adjacent to the optical fiber can also result in the diffusion of hydrogen through the polymer coatings and into the fiber core. Hydrogen which has diffused into the core of the fiber may react with core glass matrix defects, the effect of which is increased optical loss in the fiber.
Stress corrosion and hydrogen absorption can be prevented or at least reduced to a significant degree by the application of a hermetic coating to the fiber surface after fiber drawing but before any polymeric coatings are applied to the fiber surface by a variety of methods. For example, J. A. Wysocki U.S. Pat. No. 4,407,561 discloses that a variety of metals, including nickel, copper and aluminum may be used to provide a hermetic coating for a glass optical fiber. The hermetic coating is applied by passing a just-drawn optical fiber through a molten pool of metal. It is now common practice to apply a hermetic coating which includes carbon.
The problem is that these hermetic coatings typically exhibit a dark color which is difficult to hide. Conventional dyes and pigmented systems, it has been found, are unable to cover satisfactorily hermetic coating materials and still provide desired performance characteristics.
What is needed and what does not seem to be available in the prior art is a nonpigmented color coating system which may be used for optical fiber. The sought after system must be reasonably low in cost and must be capable of being applied to optical fiber along existing optical fiber manufacturing lines without necessitating any reduction in processing speed. Of course, the desired color coating system must be acceptable environmentally and must not present potential problems in handling.